Let me give an example that oversimplifies things.
Don't think of mpg, think of gallons per mile (or liters per 100km like the rest of the world does)
You normally get 30mpg, which is 0.033 GPM
Now you get 24mpg, which is 0.04 GPM
So the trailer uses 0.007 GPM.
Now your Tesla usually gets 100mpg, which is 0.01gpm
Add the 0.007gpm and you get 0.017gpm which is 59mpg...
Basically the Tesla has an efficient motor but is also very aerodynamic, so the result is probably between the two... I'm guessing 65mpge.